Technologies for integrating together, via an adhesive, heterogeneous materials in the form of a metal and a synthetic resin are demanded in a wide variety of technical fields and industries, such as automobiles, household electrical articles, industrial equipment and the like, to which end numerous adhesives are being developed on an ongoing basis. In this context, some excellent adhesives have been proposed. Adhesives that fulfill their function at normal temperature, or by heating, are used for bonding a metal and a synthetic resin into a single whole, in what is now an ordinary technology.
However, research has been conducted also on more rational bonding methods in which no adhesive is used. Methods have thus been proposed in which a light-metal alloy of magnesium, aluminum and alloys thereof, and iron alloys such as stainless steel or the like, are integrated with a high-strength engineering resin, without any intervening adhesive. Among these methods, a method that involves, for instance, inserting an aluminum alloy part into an injection mold, into which a resin component is then injected to elicit bonding (hereinafter, “injection bonding”) has just begun to be put into practice by the applicants, for combinations that include, at the time of writing, aluminum alloys and polybutylene terephthalate resins (hereinafter “PBT”), and polyphenylene sulfide resins (hereinafter, “PPS”).
As far as the applicants know, the above invention is the first invention that discloses the feature to the effect that injection bonding is possible by way of combinations of such metals and synthetic resins. The inventors have found that bonding strength (also called “fixing strength” in the present invention) increases specifically when an aluminum alloy shaped product is dipped in an aqueous solution of a water-soluble amine compound, is washed with water, is dried, and is then inserted into an injection mold, into which there is injected a thermoplastic synthetic resin composition having PBT or the like as a main component (Patent documents 1, 2, and 3). The inventors have found that a preferred surface treatment, learned from experience in liquid treatments of metals, allows stabilizing the resulting product, making it possible as a result to injection-bond a polyamide resin and an aluminum alloy. This invention was proposed in Japanese Patent Application Laid-open No. 2006-315398.
The inventors went on to develop polyamide resins to be used, with a view to finding resin compositions appropriate for injection bonding. As a result, the inventors perfected the present invention upon finding that characteristic polyamide resin compounds are appropriate for injection bonding.
[Patent document 1] Japanese Patent Application Laid-open No. 2003-200453
[Patent document 2] Japanese Patent Application Laid-open No. 2004-268936
[Patent document 3] International Patent No. WO 2004/041532
As described above, it was found that preparing a shaped product covered with ultrafine recesses, inserting the aluminum alloy shaped product into an injection mold, and injecting a polyamide resin composition into the injection mold, yields an integrated article in which there are bonded an aluminum alloy portion and a molded resin molded product. A lingering challenge was the question of whether such bonding strength between the resin portion and the aluminum alloy portion can be further increased depending on the compound of polyamide resin, among other approaches.